Access channels in Code Division Multiple Access (CDMA) systems typically include the transmission of a short signal known to the base station before the transmission of the information. This allows the base station to detect the presence of a user who has data to transmit. Because multiple mobile stations may be trying to access this channel simultaneously, each user must use a different code. These codes must be designed such that the correlator needed in the base station must be of relatively low complexity. The codes must also be robust to large differences between the carrier frequency of the received signal and the base station's receiver oscillator. They should also lend themselves to simple implementation of automatic frequency control circuits.
In the transmitter of a typical spread spectrum communication system, a source of information bits is first channel coded and then used to modulate a signal which has a bandwidth on the order of the information rate. Coherent, noncoherent, or differential modulation may be used. This signal is then spread to a wider bandwidth by multiplication with a spreading code. The signal is then translated in frequency to a carrier frequency determined by the frequency of a local oscillator and transmitted through an antenna. The signal goes through a channel where it may be modified by additive noise, changes in amplitude and phase as occurs in multipath fading, and changes in carrier frequency as when the transmitter is in motion relative to the receiver. The output of the channel is mixed to baseband with a receiver oscillator whose frequency is known by some means to be close to that of the transmitter oscillator. It is then fed to a receiver which, by some means, knows the arrival time of this signal. The receiver first despreads the incoming signal by multiplication with the same spreading code used in transmission. The signal is then demoduated and finally decoded to yield estimates of the transmitted information bits.
The processes of determining the arrival time and carrier frequency of the received signal are called time and frequency acquisition respectively. One method of performing time and frequency acquisition involves the transmission of a short training signal prior to the transmission of the information-bearing signal. There may be a period of time between the preamble and the transmission of information. This preamble also serves a second purpose in systems where the receiver does not know when a transmission will begin. In some systems, such as IS-95C and WCDMA systems, there may be a number of transmitters which periodically begin transmissions. By looking for the presence of the preamble, the receiver can determine if a transmission is about to begin and if so from which transmitter to expect further transmission.
The signal generated by the preamble generator should be easily detectable by the receiver, allow the receiver to determine the timing of the transmission with high precision and accuracy, allow the receiver to determine the carrier frequency of the received signal with high accuracy and precision, and allow the receiver to determine which transmitter is about to begin transmission.
A prior art preamble generator which generates binary signals of +1 and -1s is shown in FIG. 1. The generator consists of two orthogonal Gold code generators. The inner code, c.sub.i (n), comprises 256 chips that are modulated by 16 symbols, c.sub.o (m), corresponding to one transmitter's code, c.sub.o. The total code is therefore: ##EQU1##
The inner code is typically common to all transmitters. The preamble output from this generator suffers from several problems. First, the autocorrelation function has large peaks at multiples of 256 chips, which makes the accuracy of the estimated arrival time of the transmission poor. Second, small differences between transmit and receive oscillator frequencies cause the receiver to misidentify which transmitter is sending the preamble.
Consequently, a need exists for a method for generating preamble sequences in a spread spectrum communication system. Further, a need exists for a method for estimating the difference between the carrier frequency of the received signal and the receiver oscillator frequencies.